One construction method for aircraft wings involves creating a wing box, which forms the core of the wing. The wing box is fared into the aircraft fuselage and runs from the root towards the tip of the wing. The wing box provides the central sections of the upper and lower aerofoil surfaces for the wing, in addition to attachment points for engine pylons and control surfaces such as ailerons, trim flaps and airbrakes. The aerofoil surfaces of the wing box are provided by panels referred to as wing cover panels. The wing box includes a number of structural elements called spars and stringers, which run within the wing box structure from the root towards the tip. The spars provide the main structural component of the wing box forming the forward and aft fuel boundaries. The stringers are arranged to provide structural stability and integrity to the wing cover panels. These structural elements thus enable the wing box to provide the necessary strength for the wing during operational loading.
Wing boxes are commonly constructed from a set of discrete spars and wing cover panels, which form the walls of the wing box. Individual stringers are fastened to the wing cover panels and the wing cover panels fastened to flanges on the spars at the corners of the wing box. A consequence of this is that a large number of fixing holes are drilled in the wing cover panels and the flanges of both spars and stringers to allow bolts or rivets to be inserted. These holes are potential crack initiation sites. When building aircraft structures, one of many considerations is the balance between weight and strength. In other words, structures need to be as light as possible while providing the necessary strength and structural integrity to perform properly under operational loads. However, these weight and strength considerations have to be significantly compromised to protect the structure from any crack growth resulting from fixing holes.